A coloring agent for inkjet recording ink is mainly based on a dye, and there are disadvantages that the dye ink is inferior in water resistance and weather resistance and also easily oozes out on plain paper. In order to eliminate these disadvantages, such ink has been proposed that pigments, for example, carbon black, are used.
The pigment ink is excellent in water resistance and weather resistance and also able to record images which ooze out to a less extent but is inferior in fixing property, which is a problem. Therefore, an attempt has been made that various resins are added to improve the fixing property.
For example, Patent Literature 1 has described a recording solution which contains carbon black and a soap-free emulsion. In records obtained by using the above-described recording solution, a resin emulsion forms a film after drying of water resistance, but carbon black contained in the recording solution is dispersed by a surfactant and therefore does not have a hydrophilic group directly on the surface. There is found a disadvantage that the above-described recording solution dispersed by a surfactant is poor in dispensability due to detachment of the surfactant adsorbed on the surface of the pigment with the lapse of time and may easily increase in viscosity and cause aggregation.
Further, Patent Literature 2 has described a method in which hypohalogen is used to acid-treat carbon black. Since this method is able to introduce an aqueous dispersible functional group onto the surface by acid treatment of carbon black, the dispersion stability is better than that attained by using a surfactant. However, carbon black is known to generate humic acid due to acid treatment and the dispersion tends to decrease in pH with the lapse of time, thereby causing a decrease in dispersion stability and the deterioration of members inside an ink-used recording apparatus.
Still further, Patent Literature 3 has described an ink composition containing two or more types of resins in which a pigment that can be dispersed or dissolved in water without a dispersing agent is used. Carbon black which is contained in the above-described ink composition and can be dispersed or dissolved in water without a dispersing agent is acid-treated by using hypochlorous acid or ozone. Since there are provided no measures for the pH decrease with the lapse of time due to elusion of humic acid, the ink is not satisfactory in terms of storage stability.
In addition, Patent Literature 4 has described that attention is given to humic acid contained in acid-treated carbon black and where the concentration of humic acid is found to be one or lower in terms of the absorbance, there is provided an excellent feature of preventing clogging at a nozzle or a pen tip due to the development of solids. However, there is a problem that where humic acid is abundantly contained in a recording solution, pH is decreased with the lapse of time to deteriorate the storage stability.    [Patent Literature 1] Japanese Patent (JP-B) No. 3489289    [Patent Literature 2] JP-B No. 3405817    [Patent Literature 3] Japanese Patent Application Laid-Open (JP-A) No. 2004-204079    [Patent Literature 4] JP-A No. 11-349309